In audio applications, a speaker may be driven by an amplifier, typically a class AB amplifier. However, class AB amplifiers have low power efficiency. In portable applications, such as cell phones, personal desktop assistants (PDAs), and headphone amplifiers, battery life can be a significant concern. Also, in high-power systems, (e.g. 5 W–300 W), metal heat sinks are typically required because the low efficiency of Class AB amplifiers.
Compared to class AB amplifiers, class D amplifiers have much higher efficiency. A class D architecture could significantly increase the battery life in portable systems, and could allow multi-channel high-power systems to be designed very light and compact by eliminating the heat sinks that are typically employed.
Class D amplifiers typically employ either pulse width modulation or sigma-delta modulation. After initial conditioning of the audio signal, the signal is typically modulated by a high-frequency carrier signal. Also, the output stage of a class D amplifier is typically a pair of power MOSFETs which operate as switches to couple the output to VDD or ground based on the modulated signal. The output is generally low-pass filtered and provided to a speaker or a headphone for audio applications.